1. Technical Field
The present invention relates to an apparatus for helping to protect a vehicle occupant. In particular, the present invention relates to a seat belt webbing pretensioner apparatus which has multiple pretensioning output levels, one of which levels is provided by venting fluid pressure from the pretensioner apparatus.
2. Description of the Prior Art
A typical vehicle seat belt system includes a length of seat belt webbing wound on a spool of a seat belt webbing retractor. The seat belt webbing is extensible about a vehicle occupant to help protect the occupant. The spool rotates in a belt webbing withdrawal direction as the seat belt webbing is withdrawn from the retractor. In the event of a vehicle collision, the spool is prevented from rotating in the belt webbing withdrawal direction.
It is known to pretension, or remove any slack in, the seat belt webbing in the event of a collision. In one known seat belt webbing system, the seat belt webbing is pretensioned by moving a buckle associated with the seat belt webbing through a pretensioning stroke. The force of the pretensioning stroke is typically large enough so that, for an average male occupant, any slack in the seat belt webbing is removed. A variety of pretensioning force levels may be desirable to uniformly restrain all sizes of occupants.